The present invention relates generally to a variable speed drive for a crop harvesting machine and, more particularly, to an improved cam member for a torque sensing variable diameter power transfer assembly used in a combine harvester drive mechanism.
Torque sensing variable diameter drive mechanisms can be found, for example, in U.S. Pat. No. 3,842,637. Typically, one of the sheave sections is fixed to a power transmitting shaft to be rotatable therewith, while the other sheave section is movably mounted on the shaft to be rotatably and axially movable relative to the fixed sheave section. A first cam member is attached to the fixed sheave section and a second cam member is attached to the movable sheave section, each cam member being rotatable with the respective sheave section. The cam members are mounted so as to be in axially shiftable torque transmitting engagement with each other, while a spring biases them apart. Any rotational movement of the movable sheave section relative to the fixed sheave section due to a requirement of more power per given belt speed results in an axial movement of the movable sheave section to increase torque transmitted through the cams.
To prolong the operating life of the belt, it is desirable to place this torque sensing mechanism on the driven shaft of a power transfer mechanism. However, when a combine is traveling downhill, the power induced by gravity may cause the combine to drive the engine, i.e., the power induced by the driven shaft is greater than the power delivered by the engine's power output shaft. Under these reverse power conditions, the normally driven sheave becomes the drive sheave and the torque sensing mechanism is then effectively located on the drive shaft of the power transfer mechanism.
The use of a symmetrical cam having first and second cam surfaces at the same angle relative to the base of the cam does not provide for a proper torque transmitting property when the combine drive mechanism is operating under the aforementioned reverse power conditions, thereby allowing the belt to slip over the movable sheave section and preventing the engine from acting as a brake.